(Received 14 November 2011;accepted 1 December 2011;online 7 December 2011)

The title compound, tricaesium hepta­samarium(III) dodeca­selenide, is setting a new starting point for realization of the channel structure of the Cs3M7Se12 series, now with M = Sm, Gd–Er. This Cs3Y7Se12-type arrangement is structurally based on the Z-type sesquiselenides M2Se3 adopting the Sc2S3 structure. Thus, the structural set-up of Cs3Sm7Se12 consists of edge- and vertex-connected [SmSe6]9− octa­hedra [dØ(Sm3+ – Se2−) = 2.931 Å], forming a rock-salt-related network [Sm7Se12]3− with channels along [001] that are apt to take up monovalent cations (here Cs+) with coordination numbers of 7 + 1 for one and of 6 for the second cation. The latter cation has a trigonal–prismatic coordination and shows half-occupancy, resulting in an impossible short distance [2.394 (4) Å] between symmetrically coupled Cs+ cations of the same kind. While one Sm atom occupies Wyckoff position 2b with site symmetry ..2/m, all other 11 crystallographically different atoms (namely 2 × Cs, 3 × Sm and 6 × Se) are located at Wyckoff positions 4g with site symmetry ..m.

In the title compound, [SmSe6]9- octahedra (d(Sm3+–Se2-) = 2.8578 (9)–3.0614 (13) Å) are connected via edges and corners to form a [Sm7Se12]3- network with triple-channels occupied by Cs+ cations (Fig. 1). This network represents a defect rock-salt-type structure strongly related to that of the Z-type sesquiselenides M2Se3 (Dismukes & White, 1964) according to the formula [□]4[M]8[Se]12. In tricaesium heptasamarium(III) dodecaselenide three Cs+ cations replace one Sm3+ for charge balance. The triple-channels are arranged in a herringbone pattern and run through the structure parallel to [001]. They are filled with two crystallographically different Cs+ cations (Fig. 2). While Cs1+ exhibits a coordination number of 7+1 with an extra secondary contact (d(Cs1+–Se2-) = 3.6071 (12)–3.8053 (12) Å and 4.5421 (14) Å; Fig. 2, left), the Cs2+ cations have only six selenide anions as nearest neighbours in the shape of a trigonal prism (d(Cs2+–Se2-) = 3.5286 (16) – 3.924 (2) Å; Fig. 2, right). Owing to the very close distances between these Cs2+ cations (d(Cs2+···Cs2+) = 2.394 (4) Å) only a half-occupation of this position is possible (Fig. 2, right and Fig. 3) and stoichiometrically meaningful.

Yellow, transparent, needle-shaped single crystals of Cs3Sm7Se12 were obtained as the main product of a reaction between 0.10 g Sm, 0.08 g Se and 0.50 g CsCl added as flux and caesium source upon heating at 1073 K for 10 days in a sealed, evacuated fused-silica vessel.

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Acknowledgements

This work was supported by the State of Baden-Württemberg (Stuttgart) and the German Research Foundation (DFG; Bonn) within the funding programme Open Access Publishing. We thank Dr Falk Lissner for the data collection.

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